The butterfly knife is an old and well-known Asian knife structure. While its ancestry has been traced as far back as 800 A.D., it first surfaced in America in the early part of the Twentieth Century. It is believed that the first butterfly knives were brought to the United States by Asian immigrants and later, popularized by military personnel returning from the South Pacific after World War II. It is now one of the more popular existing knife designs as is evidenced by its frequent use in feature films such as, "Sharkey's Machine," "The Outsiders," and "Silent Rage."
In brief, a butterfly knife comprises; a blade, two isomeric handle halves, each of a thickness approximating half the blade width and each rotatably connected to the blade tang (base of the blade) with pins. The tang also features a pin (tang pin) to stop rotation of the handle halves upon blade exposing movement. Finally, the knife includes a locking latch that in one position secures the handle halves over the blade and in a second position locks the handle halves together as a handle from which the blade projects. To use the knife, the locking latch is released from the first blade covering (chambered) position. The user grips the blade half closest to the palm of the hand between the thumb and fingers and flicks the wrist. As the wrist is flicked, rotational energy is imparted to the second handle half and the blade which both rotate about the pin securing the first handle half to the tang. As the blade and second handle half move through the arc of rotation to where the blade projects 180.degree. from the chambered position (sticking straight out from the first handle half), its rotation is stopped by the tang pin. The second handle half continues to rotate relative to the tang due to its own rotatable pin connection and completes a virtually full circle to abut the first handle half. The combined movement can be fairly characterized as joint articulated rotation. Thus, in the final configuration, the blade projects from a bifurcated handle formed by the handle halves and the locking latch is locked to establish a unitary handle structure.
One of the well-known benefits of the butterfly design, believed to have contributed to its popularity, is that the two legs of the folding handle (handle halves) cover and protect the blade when the knife is in rest (chambered) position. Thus, the butterfly knife is unique in eliminating the need for a blade protecting sheath. However, the absence of a sheath creates its own problems. For example, if the user does not employ a sheath then the knife must be stored somewhere on the person. Often, such storage, if reasonably secure, detracts from and otherwise affords only marginal quick access, i.e. a pocket, a sock, etc. Not only does such placement impede access but also it reduces the speed at which the knife can be opened.